Ata (may Allah have mercy on him) said: Menstruation is fifteen days; if it exceeds that, she is a mustahadah (a woman with non-menstrual bleeding). He said: I saw Ibn Mahdi (may Allah have mercy on him) adopting the view of Ata (may Allah have mercy on him) on menstruation; he said: There is a woman among us whose menstruation lasts fifteen days.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحيض / 1540